The goal of this project is to understand the molecular mechanism of intestinal glucose and sodium transport. The intestine is responsible for the absorption of dietary carbohydrate and more than 4 liters of fluid per day. The brush border Na/glucose cotransporter (SGLT1) is responsible for absorbing 180 grams of glucose and 115 grams of sodium and is also directly or indirectly involved in the absorption of water. Oral Rehydration Therapy, credited with saving thousands of infants a day from infectious diarrhea, is based on the coupling of glucose, salt and water transport by SGLT1. The goal of this project is to understand how human SGLT1 actively transports glucose from the gut into the intestinal epithelium across the brush border membrane. Our previous studies have determined that glucose transport is driven by the sodium and electrical gradients across the brush border and now we wish to explain how glucose and sodium transport are coupled. The human SGLT1 gene will be expressed in cultured cells and biophysical methods will be used to measure the kinetics of Na/glucose co transport. These methods include electrical and optical assays to determine both the steady-state kinetics and the conformational changes of the protein that underlie coupled transport. We have determined the crystal structure of SGLTs, identified a sugar and sodium binding sites, and obtained structural insight into the active transport. The structure will be used as a guide in our biophysical experiments to determine how glucose binds and is transported through the protein molecule. Our objective is to explain the kinetics and sugar selectivity of Na/glucose cotransport in terms of the structural dynamics of SGLT1. SGLT structures will provide clues about the differences in functional properties between the brush border Na/glucose cotransport and the glucosensor (SGLT3) found in the enteric nervous system. The results will also provide a structural basis for the interaction of natural products in our diet with SGLT1 and hopefully explain how these glycosides either blunt glucose absorption or how they are absorbed. Many such glycosides, components of folk medicines, are used to treat a variety of ailments including diabetes, obesity and aging. On a broader scale our work is also relevant to the function of other members of the SGLT gene family (SLC5), e.g. the Na/iodide and SGLT2, and the structurally related serotonin reuptake transporter (SERT) in brain and enteric nervous systems.